As a polishing pad used for mirror finish of a semiconductor wafer, which is a base material for an integrated circuit, there has been frequently used, in general, a velour-like or suede-like composite material of a fiber and a resin, or a relatively flexible sheet having large compressibility prepared by impregnating a nonwoven fabric with a thermoplastic polyurethane resin and wet coagulating it.
With a recent trend toward the high integration and the multilevel interconnection, demands for a semiconductor wafer which is improved in quality such as higher flatness and which is lower priced have been increasing. Accordingly, polishing pads are also required to have higher performances such as better planarization capability than ever and to be usable for a longer time.
With respect to such a requirement, conventional nonwoven fabric type polishing pads, which are relatively flexible, show good contact with a wafer and good retention of a polishing slurry. However, because of the flexibility, they are not sufficient in the planarization of the polished surface of a wafer. In addition, the voids of the nonwoven fabric may be clogged with the polishing slurry and polishing wastes, which tends to cause the wafer surface to be susceptible to scratches. Further, the polishing slurry and the polishing waste easily penetrate deeply into the voids, which makes it difficult to brush the polishing pad, and which is the cause of the problem that the polishing pad life may be short.
On the other hand, a polishing pad using a polymer foam is also known, and is commonly used for applications, in which planarization is required, such as polishing of wafers, because of its higher rigidity than that of nonwoven fabric type polishing pads. In addition, because the polishing pad using a polymer foam has closed-cell structure, the polishing slurry and the polishing wastes do not penetrate deeply into the voids as with the nonwoven fabric type polishing pad. Therefore, the polishing pad using a polymer foam is relatively easy to dress, and hence it can stand the long-time use. In particular, a polyurethane foam is commonly used as a polymer foam, because of an excellent abrasion resistance.
The polishing pad made of a polyurethane foam is produced, in general, by appropriately grinding or slicing a polyurethane foam. Conventional polyurethane foams for a polishing pad has been produced by foam casting and curing of a two-part type curable polyurethane [see Japanese Patent Application Laid-open Nos. 2000-178374, 2000-248034, 2001-89548, Hei 11-322878, etc.]. However, with this production method, it is difficult to effect the uniform reaction and foaming, and in addition, there is a limit on the increase in hardness of the resulting polyurethane foam. Further, conventional polishing pads made of a polyurethane foam may vary in polishing performance such as the flatness of the polished surface of a wafer and the planarization efficiency. The variation in foam structure of the polyurethane foam, which is the base material, is considered to be a cause of such a problem. Besides, polishing pads with higher hardness are demanded in order to improve the planarization efficiency [see “SCIENCE OF CMP” issued by Science Forum Inc., on Aug. 20, 1997].